1. Field of the Invention
The present invention relates to a semiconductor device, a memory device, and a method for manufacturing the semiconductor device.
2. Description of the Related Art
Semiconductor devices using semiconductor elements, for example, memory devices are broadly classified into two categories: a volatile memory device that loses stored data when supply of power stops, and a non-volatile memory device that retains stored data even when supply of power stops. In terms of data retention and low power consumption, non-volatile memory devices have attracted attention.
A typical example of a non-volatile memory device is a flash memory. A flash memory includes a floating gate between a gate electrode and a channel formation region in a transistor and stores data by retaining electric charge in the floating gate. Therefore, a flash memory has advantages in that the data retention time is extremely long (almost permanent) and refresh operation which is necessary in a volatile memory device is not needed (e.g., see Patent Document 1).
However, a gate insulating layer included in a transistor is deteriorated by tunneling current generated in writing; therefore, there is a problem in that the gate insulating layer is deteriorated by repetition of a writing operation and an erasing operation and the memory element stops its function. In order to reduce adverse effects of the problem, a method in which the number of write cycles in the memory elements is the same is employed, for example. However, in order to carry this method, a complicated peripheral circuit is needed, and the problem of lifetime is not solved fundamentally. In other words, a flash memory is not suitable for applications in which data is frequently rewritten.
A typical example of a volatile memory device is a dynamic random access memory (DRAM). A memory element of a DRAM includes a read and write transistor and a capacitor, and a gate insulating layer is not deteriorated by writing operation and reading operation. However, there is a problem in that since information is stored as electric charge in the capacitor, leakage current is generated between a source and a drain even when the transistor is in an off state; thus electric charge flows into or out of the capacitor even when the transistor is not selected, so that the data retention time is short. For that reason, another writing operation (refresh operation) is necessary at predetermined intervals, and it is difficult to sufficiently reduce power consumption.
Another example of a volatile memory device is an SRAM (static random access memory). An SRAM is characterized in that it retains stored data by using a circuit such as a flip-flop and thus does not need refresh operation, but poses a problem in that an area occupied by such a memory device in a semiconductor device is large. Moreover, as in a DRAM, stored data in an SRAM is lost when supply of power stops.